1. Field of the Invention
The present invention relates to devices and methods for determining a state of polarization of an electromagnetic signal and, more particularly, to determining the state of polarization of the signal in an efficient and rapid manner.
2. Related Art
Knowledge of the state of polarization of a signal reflected from a target scene provides a powerful means of discriminating real targets from clutter and background. In general, natural backgrounds giving rise to clutter reflect electromagnetic energy with little change in polarization, while man-made objects cause depolarization of these signals.
In the same way, knowledge of the polarization of electromagnetic radiation emitted passively from a target scene provides a powerful means of discriminating real targets from clutter and background. In general, the emission polarization signatures of manmade objects will be different from that of natural objects.
Identifying the state of polarization of a signal based on determining the Stokes polarization vector components of the signal is known. In particular, a signal, such as a spectral band of light, or electromagnetic radiation in any spectral band, may be characterized as having four Stokes vector components (s0, s1, s2, and s3). The component s0 is proportional to the intensity of the wave. The components s1, s2, and s3 may be related to the orientation of the polarization, e.g., an ellipse and its ellipticity. In general, the orientation of polarization of a plane (planar phase front) monochromatic (single frequency) wave is elliptical. However, the ellipse may degenerate into a straight line in the case of linear polarization, and for circular polarization, the ellipse may degenerate into a circle. Electromagnetic radiation of broad bandwidth (polychromatic radiation) may be considered to comprise many signals each of which is monochromatic and which is generally elliptically polarized as described above.
An elliptically-polarized wave can be considered as the superposition of two waves of arbitrary orthogonal (perpendicular) polarization and amplitude a1 and a2 with phase difference δ. In this case, the components of a Stokes polarization vector (s0, s1, s2, and s3) may be equated to amplitude (a1 and a2) and phase difference (δ) as provided in the Table below: